FIELD OF THE INVENTION
This invention relates to a filter operating with surface acoustic waves--so-called SAW filters--which have one or more acoustic tracks each containing at least one input interdigital converter, at least one output interdigital converter, and at least one reflector that reflects the surface wave in the acoustic track between the input and the output digital converter by 180.degree..
SAW filters of this kind are disclosed, for example, in U.S. Pat. No. 5,379,010 to Ruile et al. (German DE-OS 39 42 140). That SAW filter has an input converter and an output converter and at least one reflector that causes the surface acoustic wave to deflect by 180.degree. on the path between the input converter and the output converter.
Filters for the mobile radio CDMA system must display a very high level of edge steepness with a relative bandwidth of, for example, 1.5%, in connection with an almost linear phase within the transmission band. Since the edge steepness for SAW filters is directly related to the length of the time window and thereby the length of a converter, it is not possible to realize such filters with a minimal chip length in the form of, for instance, a transversal filter. The advantage of a reflector, in comparison with a converter of the same length, is that, for a weighting through the round trip propagation of the surface acoustic wave, the time window made available is twice as long as that of the converter. In the case of a filter, the reflection of the reflector enters--along with the electroacoustic coupling of the input and output converter--directly into the transmission function of the filter.
Until now, flat transmission bands could only be attained with such filters using weighted converters which compensate for the convex form of the reflection function in a sub-section.